1. Field of the Invention
The present invention relates generally to cooling systems and techniques using heat pipes.
2. Background Art
When we consider the design of a cooling system, the objective is to maintain the component(s) to be cooled at a desired temperature, usually below ambient. An implementation of a typical cooling system is shown in FIG. 1. The component(s) 10 to be cooled is generally placed in an insulated chamber or housing 12, which is kept at a temperature below ambient by an active cooling device 14. The cooling device 14 can be any conventional system known in the art, such as, for example, a thermoelectric cooler or a Stirling cooler. A Stirling engine or cooler is based on the Stirling cycle, which is a well known thermodynamic cycle. The cold side of the cooling device 14 is adjacent to the component chamber 12 to absorb the undesired heat. The heat generated in the housing 12, as well as any heat gained from the higher temperature ambient, is then transferred to the cold side of the cooling device 14 (represented by arrows in FIG. 1). The cooling device then dissipates this heat plus any heat generated in the device from the hot side to the ambient.
When we consider the active cooling system design of FIG. 1, if the heat transfer from the housing 12 contents to the cold side is through conduction only, the heat flow causes a substantial temperature rise along the length of the chamber. We now consider a case where we have the components generating heat uniformly distributed along the length of a well-insulated chamber and a cooling system maintaining one end at 100° C. with the ambient at 175° C. FIG. 2 shows the resulting temperature profile along the length of the housing 12 with the chamber constructed with a highly heat conducting material. As we can see from lower plot (B), the heat flow along the body of the component chamber increases as you approach the cooling device 14 as all of the heat that is generated to the right of any given location must flow through a given cross section. As the heat flow increases, the slope of the temperature profile increases and this results in the nonlinear temperature profile seen in upper plot (A).
The objective of the cooling system is to keep the components at a temperature well below ambient and as can be seen from FIG. 2, only a very small part of the housing 12 adjacent to the cooling device 14 will be kept within the target temperature in this design. Clearly there is a need to reduce the thermal resistance along the length of the housing 12. One method to do this is to install a heat pipe along the length of the housing. The heat pipe will absorb the heat being generated by the housed components along the length of the housing and dissipate it to the cooling device.
The use of heat pipes, also know as “heat tubes”, to transfer heat is well known. Heat pipes were first suggested by R. S. Gaugler in 1942 (See U.S. Pat. No. 2,350,348) as a device to transfer heat efficiently from a hot location to a cold location. Over the years they have been used in many applications and today there are many commercial products available in the market. A more detailed description of the operation and structure of a heat pipe can be found on the World Wide Web (e.g. at http://www.thermacore.com/hpt.htm).
In the field of electronics, heat pipes have been used to transfer heat generated in electronics in a wide range of applications, including notebook PCs (See U.S. Pat. No. 6,595,269). In most of these applications the heat pipe is used as a passive device that transfers heat efficiently from a heat-generating device to an outer ambient. While most of these designs use one heat pipe to transfer the heat, a design described in U.S. Pat. No. 6,394,175 proposes the use of multiple heat pipes. In the “175 patent, the heat pipes are disposed in channels cut into a plate to which the heat dissipating electronics are mounted. The heat pipes absorb the heat from the electronics device and dissipate it at a location further away.
In other designs heat pipes are used either as passive devices to transfer the heat away or in conjunction with an active cooling device. In one design described in U.S. Pat. No. 6,052,285, a heat pipe extends from an electronic card and the condenser of the heat pipe can be inserted into a manifold that can form part of a cooling system to remove heat from the condenser. U.S. Pat. No. 6,474,074 describes an apparatus for dense chip packaging using a heat pipe in conjunction with a thermoelectric cooler and heat dissipating fins. A thermoelectric cooler, sometimes referred to as a “Peltier” cooler, is an active cooling device that transfers heat from one side to the other side when a voltage is applied to it. Another design that uses a Peltier in conjunction with heat pipes is described in U.S. Pat. No. 6,351,951. In this design, heat pipes are used to enhance the heat transfer into the cold side of the Peltier as well as to improve the heat transfer from the hot side to the ambient.
In hydrocarbon exploration and production operations, there is a need to use electronic devices at temperatures much higher than their rated operational temperature range. With oil wells being drilled deeper, the operating temperatures for these downhole instruments keeps increasing. Besides self-generated heat, conventional electronics used in the computer and communications industry generally do not have a need to operate devices at high temperatures. For this reason, most commercial electronic devices are rated only up to 85° C. (commercial rating).
Modern tools or instruments designed for subsurface operations are highly sophisticated and use electronics extensively. In order to use devices that are commercially rated in a subsurface or downhole environment, it is highly desirable to have a cooling system capable of maintaining the electronics within their operational range while disposed downhole. Conventional logging techniques include instruments for “wireline” logging, logging-while-drilling (LWD) or measurement-while-drilling (MWD), logging-while-tripping (LWT), coiled tubing, and reservoir monitoring applications. These logging techniques are well known in the art.
Heat pipes have also been implemented in downhole instruments for cooling purposes. U.S. Pat. Nos. 6,659,204, 6,378,631 and 6,216,804 describe tools for recovering subsurface core samples equipped with heat pipes. U.S. Pat. No. 4,517,459 describes a logging tool equipped with a temperature stabilization system including a heat pipe. U.S. Pat. No. 4,375,157 describes a downhole tool equipped with a thermoelectric refrigerator including a heat pipe.
There remains a need for improved cooling techniques to maintain components at a temperature below the ambient temperatures experienced in hot environments, particularly electronics housed in apparatus adapted for use where rapid temperature variations are encountered.